1. Technical Field
The invention disclosed broadly relates to transistor circuits and more particularly relates to an improved edge detection circuit.
2. Background Art
In fiber-optic communications systems, a light pulse transmitted from an information source is conducted along the optical fiber and is received by an optical detector such as a photo diode. Typically, the photo diode will generate a current pulse which then must be reliably detected in a noisy background. The current pulse generated by the optical detector is quite small, typically on the order of nanoamperes and having a duration of approximately 25 nanoseconds for typical high bandwidth communications applications. Noise sources include a dark current which arises from the reverse bias leakage current in conventional PIN diode structures. In addition, statistically occurring shot noise and thermally generated noise will also be introduced into the current waveform generated by the photo diode.
Typically, in order to reliably detect current pulses, edge detection techniques are employed wherein the current pulse is converted to a voltage pulse and is differentiated so that the positive going slope of the pulse is converted into a positive going signal and the negative going slope of the pulse is converted into a negative going signal which is output from a differentiator. The more precise occurrence of the differentiated positive going and negative going pulses allow for more precise detection of the time of occurrence of current pulse signals. The problem in the prior art has been detecting the differentiated waveform in a noisy background environment. Frequently, the amplitude of the noise pulses are almost as large as the amplitude of the differentiated positive going and negative going waveform and therefore the reliable detection of the information waveform is diminished.